Sexual Evolution and the Evolution of Sex

Sexual Evolution and the Evolution of Sex

Overview

  • Speaker: Dr. John Mayward Smith

  • Topic: "The Evolution of Sex"

Key Questions Explored

  1. Why Animals Seeking Dominance Rarely Fight to the Death?

    • The phenomenon of dominance in animal behavior often leads to displays of power rather than lethal confrontations.

  2. Parental Investment in Offspring

    • Variation in parental investment among species:

      • Some parents stay to raise the offspring.

      • Others leave the burden solely to a mate or the environment.

  3. Why Evolve Sexual Reproduction?

    • The question of why organisms choose to mate rather than reproduce asexually and invest all of their genes into the next generation.

Sexual Selection

  • Darwin's Contributions (1871):

    • In "The Descent of Man, and Selection in Relation to Sex", Darwin discusses sexual selection, noting two primary mechanisms:

      • Intrasexual Selection: Struggles among individuals of the same sex (often males) for access to individuals of the opposite sex.

      • Intersexual Selection: Preferences shown by one sex (generally females) when choosing their mates.

Intrasexual Selection

  • Mechanism:

    • Often referred to as the "law of battle", where males compete for dominance over females.

    • Alpha males often control a larger number of females.

    • Examples of Features Evolving:

      • Weapons and Size: Traits like spikes, larger body size, tusks, and aggressive behaviors are common.

    • Sexual Dimorphism:

      • Males tend to be larger and heavier than females, as seen in species like lions and elk.

Intersexual Selection

  • Mechanism:

    • Focus on the innate preferences of females when choosing mates. Traits that females find attractive lead to increased mating opportunities for males. This is sometimes called the “taste for the beautiful.”

  • Females often prefer males based on:

    • Bright colors

    • Displays of fitness (dancing, singing)

    • Indicators of health and good genes.

  • Proxy Traits:

    • Traits that signify health or genetic fitness but are not entirely accurate representations, such as colorful feathers or elaborate courtship displays.

Gamete Analysis

  • Comparison of male and female gametes:

    • Female egg size: 0.004 inch.

    • Male sperm size: 0.002 inch, with a greater quantity produced.

    • Females invest more energy since they produce fewer eggs comparatively (limited egg production).

    • Males produce between 40 million to 1 billion sperm per ejaculation, allowing for greater reproductive output.

Costs of Reproduction

  • For Males:

    • Males generally face lower costs per mating, aiming to mate with as many females as possible.

  • For Females:

    • High reproductive costs due to limited eggs, predation risk, and resource investment in offspring.

    • Females may selectively invest in males that offer direct benefits, such as resources or better survival for their chicks.

Differential Reproductive Success (R.S.)

  • Male R.S. tends to be unequal:

    • Example: In a population of fruit flies:

      • 90% of females produced offspring.

      • Only 76% of males produced offspring, illustrating variance in success.

  • Factors influencing female preferences include:

    • Colorfulness, courtship displays, parental care behaviors, and overall health.

Indirect Benefits of Mate Selection

  1. Good Genes Hypothesis:

    • Females select males that demonstrate traits indicating their fitness, leading to stronger offspring. Indirect benefits include better health, improved foraging skills, and superior predation protection.

  2. Direct Benefits:

    • For instance, in scorpionflies, females prefer males that present larger nuptial gifts, which is a way of assessing male fitness based on resources provided.

Examples of Sexual Selection Traits

  1. Pronghorn Antelopes:

    • Females search for males that provide optimal predator protection.

  2. Bison in Montana:

    • Larger males are preferred, leading to offspring with enhanced growth rates and survival rates.

  3. Golden Peasants:

    • Example of ornamental traits influencing mate choice despite potential survival costs.

Quiz Review

  1. Name & ID signature required

  2. Identify one cost for females regarding gamete production.

  3. Provide two features of males relevant to intrasexual selection. Examples include horns, claws, etc.

  4. List two male traits valued by females in intersexual selection. Examples include color and performance, as well as nest-building capabilities.

Conclusion

  • The discussions highlighted the complexity of sexual evolution mechanisms such as intrasexual and intersexual selection, demonstrating the balance between reproductive strategies and mate choice in varying ecological contexts.